This disclosure relates generally to a method for color finishing aluminum or aluminum alloys, and, more particularly, to a method for providing a color-anodized finish on an aluminum- or aluminum alloy motor vehicle frame or component.
Aluminum and aluminum alloys are generally classified with a four-digit system that is based upon the principal alloying element. For example, Group 5000 generally refers to aluminum alloys that contain magnesium as the principal alloying additive whereas Group 6000 series refers to aluminum alloys that contain both magnesium and silicon as the principal alloying additives.
Aluminum motor vehicle frames and components are typically subjected to an electrostatic coloring process that provides desirable decorative effects as well as resistance to the corrosion as a result of exposure to harsh environmental conditions. Most often, a voltage differential is applied across a surface to be colored and the surface is sprayed with an electrostatic paint. Negatively charged atomized paint particles and a grounded workpiece create an electrostatic field that draws the paint particle to the workpiece, minimizing overspray. As the coating is deposited onto the workpiece, the charge dissipates through the ground and returns to the power supply completing the circuit. A protective topcoat is applied to the painted surface in a similar manner to maintain the integrity of the paint and to provide gloss effects to the finished surface.
Spraying of paints and protective topcoats, however, generally results in a substantial waste of material. Even if the distance between the spray head and the surface to be coated is minimized, errant paint particles may become deposited on surfaces other than those for which the particles are intended. In such a case, the surface coatings may be non-uniformly deposited, particularly at the contours of the surface. The non-uniform deposition of the coatings, as well as other irregularities in the coating process, may provide variations in finish quality. Moreover, surface imperfections are possible due to air molecules becoming trapped in the coating surface.
Another drawback to electrostatic finishing, particularly for coating complicated surfaces, is the Faraday cage effect, which is characterized by a tendency for charged coating particles to deposit a round entrances of cavities. The Faraday cage effect allows electric charges on a conductor to reside on the outer surface of the conductor. In the case of coating complicated surfaces, the electric charge resides on the entrances of cavities. High particle momentum can help overcome Faraday cage effects, because particles with greater momentum (larger particles or particles traveling at higher speeds) are influenced less by the electrostatic forces. However, high particle momentum also lowers efficiency.
Furthermore, even if the amount of over spray is minimized during the spraying processes, the paints and topcoat compounds are atomized, which causes the compounds to be airborne for periods of time beyond which may be necessary. The presence of airborne particles generally provide a basis for environmental concerns as well as concerns related to particle inhalation by operators.